Re: [boost] [type_erasure]bug in computed_bases?

Subject: Re: [boost] [type_erasure]bug in computed_bases?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-08-29 19:23:50

On 08/28/12 07:22, Larry Evans wrote:
[snip]
> Now, the following questions concern the test code (compute_bases.cpp)
> attached to my previous post. The output of that test code shows that
> one of the superclasses of any<requirements_t> is:
>
>   concept_interface
>   < requirements_t
>   .
>   .
>   .
>   >
>
> However, I can't think of a use case for providing such a superclass.
> The unspecialized concept_interface definition:
>
> template<class Concept, class Base, class ID, class Enable = void>
> struct concept_interface : Base {};
>
> has an empty body; hence, the only use of putting such a
> concept_interface into one of the superclasses of an any (as opposed
> to just using Base) is to allow for a specialization of it.  That's
> completely unexpected to me, and I suspect, to others.  Also, the
> Concepts in Depth tutorial:
>
>
http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/doc/html/boost_typeerasure/concept.html
>
> provides no examples where the 1st template argument to
> concept_interface is an mpl sequence of other concepts, leading the
> casual reader to jump to the conclusion that the Concept arg to
> concept_interface is *not* an mpl sequence.
>
> Could you please provide a use case for such a concept_interface
> specialization so users could understand when it might be useful?
[snip]
iterator.hpp provides such a use case:

template<class T, class Reference, class DifferenceType, class ValueType>
struct iterator< ::boost::no_traversal_tag, T, Reference,
DifferenceType, ValueType> :
    boost::mpl::vector<
        copy_constructible<T>,
        constructible<T()>,
        equality_comparable<T>,
        dereferenceable<typename iterator_reference<Reference,
ValueType>::type, T>,
        assignable<T>
    >
{
    typedef ValueType value_type;
    typedef typename iterator_reference<Reference, ValueType>::type
reference;
    typedef DifferenceType difference_type;
};

template<class T, class Reference, class DifferenceType, class ValueType>
struct iterator< ::boost::incrementable_traversal_tag, T, Reference,
DifferenceType, ValueType> :
    boost::mpl::vector<
        iterator< ::boost::no_traversal_tag, T, Reference, DifferenceType>,
        incrementable<T>
    >
{
    typedef ValueType value_type;
    typedef typename iterator_reference<Reference, ValueType>::type
reference;
    typedef DifferenceType difference_type;
};

template<class T, class Reference, class DifferenceType, class ValueType>
struct iterator< ::boost::forward_traversal_tag, T, Reference,
DifferenceType, ValueType> :
    iterator< ::boost::incrementable_traversal_tag, T, Reference,
DifferenceType, ValueType>
{};

template<
    class T = _self,
    class Reference = boost::use_default,
    class DifferenceType = std::ptrdiff_t
>
struct forward_iterator :
    iterator<boost::forward_traversal_tag, T, Reference, DifferenceType>
{};